As a teacher, I understand that choosing the right version of the test will depend on what you want to achieve. Are you looking for a paper from Oracle that says you did good and understand the language? Then choose the one that you feel more comfortable with. Do you want employers to see you are up to date in the Java world? Choose 8.

Finally, the one I like the most, do you want to be a good Java developer and understand the language decisions and its implications behind the scenes? To understand why other language decisions might be good or bad and why we do things the way we do in Java? And not start coding like 1990's while in the 2010s? Then study and try out all examples for the Java 8 version. You will be up to date and also understand what is good and what is bad, why and when. That makes a huge difference in your every day developer life.

No, seriously now. Since Java 7, Oracle has split the certification in 2 parts: the OCA one is the entry-level exam and covers java and OO basics. The OCP is the more advanced exam and covers a lot of advanced topics (generics, threads, concurrency, jdbc, file i/o, nio2.0,...). So OCPJP6 covers Java & OO basics together with a bunch of advanced topics (generics, threading, concurrency, inner classes,...).
The OCAJP8 exam covers Java & OO basics, including some new language features like a few classes of the new Date & Time API and lambda expressions. You'll find all exam objectives here. To get on the same level as an OCPJP6 certified developer you'll need to pass the advanced OCPJP8 exam as well. This exam covers besides the advanced topics of OCPJP6 also JDBC, NIO2.0, resource bundles, and of course the new language features of Java 8 like streams and lambdas.